Fluid servo systems are used for many purposes, one being to position the flight control surfaces of an aircraft. In such an application, system redundancy is desired to achieve increased reliability in various modes of operation, such as in the control augmentation or electrical mode. In conventional electro-hydraulic systems, plural redundant electro-hydraulic valves have been used in conjunction with plural redundant servo actuators to assure proper position control of the control surface in the case of failure of one of the valves and/or servo actuators. Such added redundancy, however, results in a complex system with many additional electrical and hydraulic elements necessary to perform the various sensing, equalization, timing and other control functions. The system's complexity has reduced overall reliability, has increased package size and cost, and has imposed added requirements on the associated control electronics.
An alternative approach to the electro-hydraulic control system is the electro-mechanical control system wherein a force motor is coupled directly and mechanically to the main control servo valve. In this system, system redundancy has been accomplished by mechanical summation of forces directly within the multiple coil motor as opposed to the conventional system where redundancy is achieved by hydraulic force summing using multiple electro-hydraulic valves. If one coil or its associated electronics should fail, its counterpart channel will maintain control while the failed channel is uncoupled and made passive.
Various force motor configurations have been developed for directly driving the main servo valve in the control system. Basically, the force motor output is in the form of either linear or rotary motion. Linear motion obviously may be directly coupled to a sliding-type valve whereas rotary motion is more compatible with a rotary-type valve. For various reasons, it may be desirable to mate a force motor having a rotary output motion to a sliding-type valve. Accordingly, it would be advantageous and desirable to have a drive mechanism that couples a rotary force motor to a sliding-type valve.
It also would be advantageous and desirable to provide a rotary-to-linear drive mechanism that accommodates additional control inputs. This is particularly desirable in those servo systems wherein a manual input to the main servo valve is provided in the event that a mechanical reversion is necessary after multiple failures have rendered the electrical mode inactive. In known servo systems of this type, the manual input may operate upon the spool of the main servo valve whereas the electrical input operates upon a movable sleeve in the main servo valve. Upon rendering the electrical input inactive, it is necessary to move the valve sleeve to a neutral or centered position and to lock it against movement relative to the valve spool controlled by the manual input. Heretofore, this has been done by using a centering spring device which moves the valve sleeve to its centered or neutral position and a spring biased plunger that engages a slot in the valve sleeve to lock the latter against movement. The plunger normally is maintained out of engagement with the slot during operation in the electrical mode by hydraulic system pressure, and may have a tapered nose that engages a similarly tapered slot in the valve sleeve to assist in centering the valve sleeve. Such centering and locking arrangement, however, is subject to several drawbacks. For instance, in the event a chip or some other obstruction becomes lodged between the valve spool and sleeve or otherwise a high friction condition should occur therebetween, substantial reactive forces may be applied through the manual input path to the sleeve which may result in unseating of the plunger which in turn would render the manual mode inoperable.
Drive mechanisms for converting rotary motion to linear motion at valves for controlled operation of hydraulic actuators also are known. In U.S. Pat. No. 3,636,779, issued Jan. 27, 1972, such a mechanism is shown for controlling the elevation and azimuth of the turret of an ordnance vehicle. This mechanism utilizes an angular or radial contact type bearing to transmit a torsional moment from a rotating shaft to a nutating sleeve. The rotating shaft has a canted land on which the inner race of the bearing is fitted. The outer race of the bearing is fitted in the sleeve which has a radially extending guide shaft. The guide shaft extends into a guide slot therefor that constrains the guide shaft to arcuate movement in a plane passing through the rotating shaft's rotational axis. This accordingly causes the sleeve and guide shaft to wobble or nutate upon rotation of the rotating shaft. A linkage connected between the guide shaft and sliding valve provides for linear movement of the valve in response to the nutating movements of the sleeve and guide shaft.
While the valve drive mechanism disclosed in U.S. Pat. No. 3,636,779 may find particular utility in an ordnance vehicle, its use in an aircraft flight control system may not be particularly desirable for various reasons. For instance, the high resultant forces developed in the angular contact type bearing from the applied torsional moments limit the load carrying ability of the mechanism. Such high resultant forces also tend to spread apart the inner and outer races which has an adverse effect on the mechanism's stiffness. Moreover, to achieve control of bearing backlash or preload, the angular contact bearing must be specially fabricated or selected with no adjustments being available therein. In addition, such mechanism utilizes a semiflexible linkage to avoid undesirable valve plunger side loads and associated friction, which further adversely affects the mechanism's stiffness and load carrying ability.